Among the different processes the cork undergoes, washing definitely is the one that affects mostly its performance in the bottle. Any washing should have as objectives the physical cleaning of the cork, its chemical sterilization and the reduction or elimination of any risks associated with cork releasing heavy molecules or volatile compounds in the wine.
As the recommendations and fundamental principles of the CeLiege Code suggest, bad practices and bad washing techniques, apart from not reaching the objectives, can cause the following counter-effects:
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leave oxidative residues in the cork;
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leave chemical residues on the cork;
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contaminate the cork;
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destroy the cellular walls of the cork.
After several years of research and analysis, the R&D department of Colombin developed a successful method for the extraction of volatile substances from cork. This system, called “PRESTERIL”, has been applied since 1996 to all the corks produced by Colombin, effectively making them wine-sterile.
The PRESTERIL washing method achieves the following goals:
The elimination of residues of peroxides is obtained by the prevention of contact between hydrogen peroxide and corks. The peroxide is first dissociated and then put in contact (in ionic form) with the corks. In this way, as no hydrogen peroxide molecules are present, the oxidation of double bonds of unsaturated aliphatic compounds with the formation of epoxides is prevented. As known, this reaction is steril-controlled and in order to occur it needs a molecule of hydrogen peroxide in its spatial structure. The reactivity of these compounds with sulphur dioxide can cause the premature ageing of wines.
The elimination of volatile compounds is mostly aimed at the reduction or elimination of substances that cause anomalous smells and off flavours. With the PRESTERIL washing it is obtained by distillation of the corks (by exposing them to a current of steam). The steam causes a high humidification of cork. The excess water is eliminated through a thermal treatment at 130±5°C for 3 hours. During this time the corks are hit by a moderately humid air current that creates a real distillation in steam current of the volatile compounds of the corks.
The PRESTERIL treatment provides not only a surface disinfection of the corks, but a truly deep sterilization, as it penetrates into the lenticels up to the inner walls of cork cells through a cleansing cycle consisting of washing, rinsing and air circulation. Such result, together with the elimination of substances that can serve as breeding ground for moulds, yeasts and bacteria and with the relatively low residual humidity, make the re-contamination of corks very difficult, if these are kept in good conditions and in a clean environment.
Moreover, the PRESTERIL process uses as components hydrogen peroxide and a inorganic catalyst. These two products are instable at high temperatures and decompose completely during the process. The output of gases in the process amounts to few ppm and the PRESTERIL process does not produce waste water and is therefore ecological.
Needless to say, the cork used in the process needs to be of a very high grade and structurally sound to be able to withstand the high temperatures involved. Moreover, thanks to the combination of moisture, temperature and steam, the process acts delicately on the corks and does not damage their cellular structure, thus not compromising their performance in the bottle.